1. Field of the Invention
The present invention relates to the hydrogenation of benzene to produce cyclohexane. More particularly the invention relates to a process wherein the hydrogenation of the benzene and separation of the product by distillation is carried out simultaneously in a distillation column reactor.
2. Related Information
Cyclohexane is the main precursor for the production of nylon products and as such the demand remains strong. Cyclohexane was first obtained by the direct fractional distillation of suitable crude petroleum refinery streams. Now the major portion of cyclohexane is obtained from the direct hydrogenation of benzene. Conventionally the reaction is carried out in vapor or mixed phase using a fixed bed reaction. The reactor temperature is controlled to be between 350 to 500.degree. F. Higher temperatures can lead to thermodynamic limitations on benzene conversion, thermal cracking and increased by-product.
Peterson in U.S. Pat. No. 2,373,501 discloses a liquid phase process for the hydrogenation of benzene to cyclohexane wherein a temperature differential is maintained between the top of the catalyst bed where benzene is fed and the outlet where substantially pure cyclohexane is withdrawn. The temperature differential is due to the change in the exothermic heat of reaction released as less and less benzene is converted as the concentration of benzene decreases. Specifically the top of the catalyst bed is at a higher temperature than the lower catalyst bed. Hydrogen is supplied counter current to the benzene/cyclohexane flow. Temperature control coils are disposed within the reactor to maintain the temperature differential if the exothermic heat of reaction is not sufficient or to cool the bed if too much heat is released. Peterson recognizes that although the bulk of his reaction takes place in the liquid phase a portion of the benzene and cyclohexane will be vaporized, especially near the top of the reactor where the benzene concentration is highest and conversion is highest. A reflux condenser is provided to condense the condensible material and return it to the reactor. Thus, a substantial portion of the heat of reaction is removed by condensation of the reactants vaporized throughout the reaction. Peterson maintains a liquid level above the topmost catalyst bed but allows room for vapors to escape to the condenser where the heat of reaction is removed.
Larkin, et al. in U.S. Pat. No. 5,189,233 disclose another liquid phase process for the hydrogenation of benzene to cyclohexane. However, Larkin, et al utilize high pressure (2500 psig) to maintain the reactants in the liquid state. In addition Larkin, et al disclose the use of progressively more active catalyst as the concentration of benzene decreases to control the temperature and unwanted side reactions.
Hui, et al. in U.S. Pat. No. 4,731,496 disclose a gas phase process for the hydrogenation of benzene to cyclohexane over a specific catalyst. The catalyst reported therein is nickel supported on a mixture of titanium dioxide and zirconium dioxide.
The hydrogenation of benzene is also useful to remove that aromatic compound from gasoline streams. One example of this process is disclosed by Hsieh, et al in U.S. Pat. No. 5,210,348 wherein hydrogenation of the benzene fraction is used alone or in combination with alkylation. The hydrogenation of the benzene is disclosed to be in a standard single pass fixed bed reactor. In some schemes for the reduction of aromatic compounds in gasoline the ASTM D-86 90% point is specified such that the aromatic and unsaturated cyclic and polycyclic compounds are precluded from the gasoline blending pool. This has been termed a T-90 gasoline stock having a desired ASTM 90% point. The resultant T-90+ bottoms which are largely unsaturated cyclic and polycyclic compounds must be disposed of and hydrogenating them to produce lighter more saturated compounds for the gasoline pool is an attractive alternative.
A typical problem with the hydrogenation of benzene to cyclohexane is the competing reactions. Particularly isomerization to methyl cyclopentane is unwanted. Additionally at higher temperatures cracking of the ring occurs producing undesirable C.sub.5 and lighter products. U.S. Pat. No. 5,773,670 discloses a process wherein unsaturated cyclic and polycyclic compounds (particularly benzene) are hydrogenated. In the process disclosed therein the hydrogen and unsaturated cyclic and polycyclic compounds are fed together as one stream below the catalyst bed in the distillation column reactor. In addition, to achieve complete conversion of the benzene to cyclohexane a polishing reactor was necessitated.
U.S Pat. No. 5,856,602 discloses the hydrogenation of a selected aromatic compound contained in a naphtha stream by feeding the naphtha stream and hydrogen to a distillation column reactor below the bed containing the catalyst.
It has been found that a downflow catalytic distillation reactor, that is, one in which the benzene containing stream is fed above the catalyst zone provides very high conversion. The major by product in conventional reactions is methyl cyclopentane, which is not present in the present reaction.